> allow the creation of a new FAT-32 image file of a specified length given a target directory [emphasis added] I think that's your problem: file systems are s

> allow the creation of a new FAT-32 image file of a specified length given a target directory [emphasis added] I think that's your problem: file systems are supposed to go onto a preexisting block device, which you'll need to create seperately. For example, on my system: dd if=/dev/zero of=/tmp/foo.dos bs=512 count=128K mkdosfs -F 32 /tmp/foo.dos Once you've created the filesystem, fill it with: mkdir /tmp/foo/ sudo mount -t msdos -o loop /tmp/foo.dos /tmp/foo/ # dump stuff into /tmp/foo/ sudo umount /tmp/foo.dos # or /tmp/foo/ # filesystem is in /tmp/foo.dos